The present invention relates, in general, to electronic devices and, more particularly, to a conductive enclosure for an electronic device.
Micro-electronic devices are typically placed in an enclosure to protect them from adverse environmental elements. One specific type of electronic device that is placed in an enclosure during manufacture is a capacitive sense accelerometer. One such prior accelerometer uses a sensing element formed from three parallel polysilicon layers that are formed overlying a supporting substrate. The accelerometer is enclosed and hermetically-sealed from the environment by an overlying cap. The middle polysilicon layer provides a middle plate that acts as a seismic mass and that is free to move in response to acceleration. In the absence of external forces, the middle plate is centered between the other two polysilicon plates resulting in equal top and bottom capacitance values. In the presence of an external acceleration, the middle plate moves, resulting in a differential capacitance change, from which the driving acceleration can be determined. Such a sense accelerometer is described by Ristic et al. in "A Capacitive Type Accelerometer with Self-Test Feature Based on a Double-Pinned Polysilicon Structure" 7th International Conference on Solid-State Sensors and Actuators: Transducers 1993, Yokohama, Japan, Jun. 7-10, 1993, Digest of Technical Papers, pp. 810-813, which is hereby incorporated by reference in full.
A previously unrecognized problem that has been found with the prior accelerometer discussed above is that its output sensitivity is reduced due to parasitic capacitances from the top and bottom capacitors of the sensing element to the enclosing cap and underlying substrate. The presence of these parasitic capacitances has been found in some cases to reduce the accelerometer's sensitivity by about fifty percent. Accordingly, there is a need for an accelerometer enclosure that reduces the adverse effects of the parasitic capacitances of the supporting substrate and enclosing cap.